Geology and Petrology. Xxxx X. Xxxxxx, Xxxxx X. Xxxxxxx-Xxxxxx, and Xxxxxx X. Xxxxx Geological Survey of Alabama Much of the effort last quarter focused on characterization and correlation of well logs in the 4-square-mile area containing the test site at the center. All of these well logs have now been digitized. Most xxxxx have SP-resistivity logs, and where these logs are unavailable, gamma-neutron logs were digitized in their place. The digital logs were used to construct a network of 19 stratigraphic cross sections using the base of the Ferry Lake Anhydrite as a datum. Sandstone-conglomerate units in the upper Xxxxxxx Sand were correlated from the 12 Sand through the 20-A Sand. The cross sections demonstrate the extreme facies heterogeneity of the upper Xxxxxxx, and this heterogeneity is well expressed within the five-spot well pattern where the field test will be conducted. Of particular interest is the 16-2 Sand, which is interpreted as a composite of two tiers of channel fills. Pay strata are typically developed in the lower tier, and this is where CO2 will be injected. The upper tier is highly heterogeneous and is interpreted to contain sandstone fills of variable reservoir quality, as well as mudstone plugs. In the northwest corner of the five-spot (Well B-19-7), interestingly, the pay zone is in the upper tier, thus the degree of hydraulic communication with the main pay zone needs to be determined to understand the effects of reservoir heterogeneity on the performance of CO2 injection operations.
Geology and Petrology. Stratigraphic cross sections constructed from digitized well logs demonstrate the extreme facies heterogeneity of the Upper Xxxxxxx Sand. Of particular interest is the 16-2 Sand, a target of the CO2 test injection, which is interpreted as a composite of two tiers of channel fills. Pay strata are typically developed in the lower tier, where the CO2 will be injected. The upper tier is highly heterogeneous and is interpreted to contain sandstone fills of variable reservoir quality, as well as mudstone plugs. In the northwest corner of the five-spot (Well B-19-7), the pay zone is in the upper tier, thus its degree of hydraulic communication with the main pay zone needs to be determined in order to understand the effects of reservoir heterogeneity on the performance of the CO2 injection. Analysis of cores and thin sections indicates that the composition and reservoir quality of the upper Xxxxxxx Sand reflects diverse processes driven by alternating episodes of subaerial exposure and marine flooding, as well as burial diagenesis. Subaerial exposure led to formation of oxidized paleosols and erosional relief that in places may have approached 20 meters. Sand bodies were preserved primarily during inundation of the erosional landscape, and trace fossil assemblages and calcareous faunas consisting of oysters, foraminifera, and algae indicate that most sandstone bodies have been reworked by marine processes. Pore-filling clay adversely affects reservoir quality and apparently formed as sediment was homogenized during burrowing and by infiltration during subaerial exposure. Exposure and marine flooding further contributed to oxidation of sandstone bodies, dissolution of feldspar and other labile grains, and precipitation of carbonate cement. Cementation continued during burial and culminated in precipitation of pore-filling dolomite cement prior to petroleum entrapment. This work represents significant revision of the conventional geologic model for the field, received from early studies of the formation in the 1950's and 1960's, and has significant bearing on the approach to the test injection and interpretation of its results.
